The present invention relates to a motor controller for opening and closing a switching element arranged on a power supply line to a motor by a PWM control signal, for controlling the output of the motor utilized, for example, for generating steering assist power in an electric power steering apparatus.
In a motor controller, which performs a feedback control of the motor output in accordance with the armature current flowing through a coil constituting the armature winding of the motor, the current flowing through a current detection resistor arranged in a motor drive inverter circuit is detected as the armature current. Applied voltage to the coil constituting the armature is changed to control the motor output by opening and closing operation the switching element, which constitutes the inverter circuit, in accordance with the PWM control signal having a duty cycle corresponding to a deviation between a target current and the detected current.
When the opening and closing operation of the switching element is performed by the PWM control signal, optimal current detection is not always possible due to the existence of a period of time during which the current does not flow into the current detection resistor. For this reason, the duty cycle of the PWM control signal corresponding to the applied voltage to the coil must be restricted, causing the availability of voltage of the power supply source to drop and impeding the improvement of motor performance.
Accordingly, a method for calculating the armature current based on the speed electromotive force, the drive voltage, the winding resistance, and the inductance of the motor without using any current detection resistor has been proposed (Japanese Laid-open Patent No. 2001-268980).